Process for reducing the thermal decomposition of molten polyhexamethylene-adipamide



United States Patent M PROCESS FOR REDUCING THE THERMAL DE- COMPOSITKON 0F MOLTEN POLYHEXAMETH- YLENE-ADIPAMIDE Heinz Indest, Obernburg (Main), Germany, assignor to Vereinigte Glanzstofi-Fabriken AG., Wuppertal-Elberfeld, Germany No Drawing. Filed July 14, 1961, Ser. No. 123,936 Claims priority, application Germany July 14, 1960 2 Claims. (Cl. 264-176) This invention relates to the treatment of polyarnides, particularly polyhexarnethylene-adipamide, better known as nylon in which the polyamide is melted and extruded into shaped articles such as films, filaments, ribbons, tubes and the like. The invention is especially concerned with a method or means of avoiding or at least substantially reducing the thermal decomposition of the polyhexamethylene adiparnide as it is processed in its molten form.

In the production of shaped articles of polyamides such as nylon, especially in making fibers or filaments, the polymer is first melted and then extruded through spinning nozzles or extrusion orifices in the same apparatus. It is well known that polyhexamethylene-adipamide is decomposed by the accompanying high tem peratures which must be in excess of about 263 C. Such decomposition will cause discoloration of the shaped article and tend-s to reduce its desirable physical properties. In order to avoid or at least reduce this decomposition, a number of procedures have been followed in the past. For example, extreme care is taken so that the polymer is subjected for only a short period of time to the high temperature necessary for maintaining it in its molten state. Also, heat stabilizing additives are often added to the molten polymer although these have not been particularly effective in the actual process ing of the molten material.

The apparatus employed for melting and extruding polyhexamethyleneaadipamide has a number of conventional forms and essentially includes a zone in which polymer particles or granules are melted so as to flow through a spinning device. Thus, it is common practice to melt the polymer granules on a heated grid or grate and then force the molten polymer by means of a metering pump through a spinning or extruding vessel. The spinning pump and nozzles or orifices are generally referred to as the spinning head while the heated grid can be contained within a spinning vessel arranged above the spinning head. The apparatus is ordinarily constructed of steel and in some cases steel alloys have been employed.

It was believed that the metal construction of the apparatus tended to have a catalytic effect upon the thermal decomposition of the molten polymer, and for this reason, an attempt to avoid such decomposition has been made by coating the inner walls of the spinning vessel and the spinning head with a silicone film. For example, a cross-linked methyl phenyl silicone has been employed as a coating material for this purpose. While it is true that apparatus coated with such a silicone layer provides a reduction of the thermal decomposition of the melt, it is disadvantageous because the coating does not adhere to the walls of the apparatus for a sufiicient period of time. The constantly flowing molten polymer rubs the silicone film away from the walls of the apparatus, and it is necessary to repeatedly clean the apparatus and renew the silicone coating.

In the development of the present invention, it was found that various heavy metals do have a pronounced catalytic eifect so as to increase the rate at which the molten polymer is thermally decomposed. Iron particu- 3,121,763 Patented Feb. 18, 1964 larly exhibits this catalytic effect, but steel alloys such as V2A or V4A steel also promote the thermal decomposition. Thus, decomposition of the polymer is caused both by the time during which the melt is in contact with the apparatus, the particular metal employed and also the total surface area of those metal parts which are in contact with the melt.

in accordance with the present invention, it has now been found that the thermal decomposition of polyhexamethylene-adipamide during its spinning or extrusion in the molten state can be considerably reduced if those surfaces of the apparatus which come in contact with the molten polymer are coated or plated with chromium or a chromium alloy. The process of the present invention is thus carried out by melting the polyhex-amethyleneadipamide polymer and pumping it through an enclosed zone for extrusion therefrom such that the molten polymer is in contact with chromium or chromium alloy metal surfaces.

One advantage of this invention is that the apparatus need not be constructed of special steel alloys. Also, to the extent that other process conditions permit, cheap steels or less expensive metals or alloys can be used for manufacturing the spinning vessel and spinning head. The chromium plating of the inner walls or other exposed parts of the apparatus can be accomplished by well known methods. Thus, in addition to the heated grid or grate and inner walls of the spinning vessel, the exposed parts of the spinning or metering pump can also be provided with a chromium or chromium alloy plating. Such plating provides a coating which cannot be rubbed away and is therefore considerably more advantageous than the above mentioned coating of a silicone film which is rubbed off in a very short period of time.

It was quite surprising to discover that chromium or chromium alloys do not have the same catalytic effect in thermally decomposing the polymer as do other commonly employed metals.

As an illustrative example of the invention, conventional polyamide spinning apparatus constructed of steel parts was disassembled and those parts normally exposed to the molten polymer were then chromium plated. The apparatus was then reassembled and employed in the usual manner for melting and spinning filaments of polyhexamethylenewadipamide. The resulting filaments exhibited considerably less thermal decomposition than those which are obtained for unplated but otherwise identical spinning apparatus. 1

In addition to its lack of catalytic effect, the chromium plating employed for the purposes of this invention is especially advantageous because of its durability and its relatively low coefiicient of friction in processing the polyamide melt. In addition, the chromium plated apparatus is more easily cleaned and requires only a minimum amount of time for maintenance. While chromium is preferred as the plating material, well known chromium alloys are also suitable which contain at least about 60% 'by weight of chromium and one or more other metals such as nickel and smaller amounts of molybdenum, manganese and under 5% iron.

The invention is hereby claimed as follows:

1. In a process for the extrusion of a molten polyhexamethylene-adipamide polymer into a shaped article, the improvement for reducing the thermal decomposition of said polymer which comprises melting said polymer and pumping it through an enclosed zone for extrusion therefirom such that the molten polymer is in contact with metal surfaces of a metal selected from the group consisting of chromium and alloys of at least 60% by weight of chromium and at least one other metal selected from References Cited in the file of this patent UNITED STATES PATENTS 2,496,961 Schneider Feb. 7, 1950 4 Oresswell July 18, 1950 Pace May 6, 1952 Hull Jan. 3, 1956 FOREIGN PATENTS Netherlands Apr. 15, 1950 

1. IN A PROCESS FOR THE EXTRUSION OF A MOLTEN POLYHEXAMETHYLENE-ADIPAMIDE POLYMER INTO A SHAPED ARTICLE, THE IMPROVEMENT FOR REDUCING THE THERMAL DECOMPOSITION OF SAID POLYMER WHICH COMPRISES MELTING SAID POLYMER AND PUMPING IT THROUGH AN ENCLOSED ZONE FOR EXTRUSION THEREFROM SUCH THAT THE MOLTEN POLYMER IS IN CONTACT WITH METAL SURFACES OF A METAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM AND ALLOYS OF AT LEAST 60% BY WEIGHT OF CHROMIUM AND AT LEAST ONE OTHER METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, MOLYBDENUM, MANGANESE AND LESS THAN 5% BY WEIGHT OF IRON. 